Our view has developed to consider phenylethanolamine N-methyltransferase (PMNT)-containing neurons a special subset of noradrenergic neurons. These cells innervate the locus coeruleus as well as preganglionic fibers of the adrenal medulla and appear to play important regulatory roles in sensory transduction, as well as maintenance of homeostatic mechanisms in response to a variety of physiological stimuli. Lesion studies indicate that the bulk of PNMT resides in some cell type other than neurons. Immunohistochemical techniques have been combined with primary astrocyte culture to examine the presence of PNMT-like immunofluorescence in a subset of astrocytes in culture derived from the rat hypothalamus, cortex and brainstem. A radiometric assay has been used to demonstrate significant norepinephrine N-methylating activity in cultured astrocytes. Astrocytes isolated from adult rat brain by differential and discontinuous density centrifugation have been examined by immunohistochemical techniques to identify PNMT.